Conventionally, for example, ball joints used in suspension devices or steering devices of automobiles have been each constructed by slidably attaching a ball stud provided with an approximately globular ball portion to a synthetic resin-made ball seat being a bearing seat provided in a socket. The ball seat has had an approximately spherical sliding surface along the outer circumferential surface of the ball portion. In addition, a grease being a lubricant has been filled between the outer circumferential surface of the ball portion and sliding surface of the ball seat (See for example Japanese Laid-open Patent Publication No. 2000-230540 (Pages 4-6, FIG. 1)).